Liquid crystal display device

ABSTRACT

There is disclosed a liquid crystal display device having two substrates which are placed opposite to each other with a liquid crystal material sandwiched therebetween. A phase plate is formed on the liquid-crystal side surface of one of the substrates. The phase plate is covered with color filters.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device and,more particularly, to a liquid crystal display device having a phaseplate.

2. Description of the Related Art

In some known reflective liquid crystal display devices, phase plates orphase shifters are disposed on surfaces located opposite to theliquid-crystal sides of substrates which are disposed opposite to eachother with a liquid crystal material sandwiched therebetween.Furthermore, a structure in which phase plates are disposed on theliquid-crystal sides of substrates and thus the phase plates areincorporated within a liquid crystal display device is known, asdisclosed in JP-A-9-146124 (and corresponding U.S. Pat. No. 6,016,178).

SUMMARY OF THE INVENTION

In the liquid crystal display device of the structure described above,each phase plate is formed on the upper surface of a color filter anddesigned as a layer underlying an orientation film.

It has been confirmed, however, that the phase plates in this structurecannot fully exhibit their functions. Research into the cause hasrevealed that when the orientation film is formed after formation of thephase plates, the used liquid solution melts the surface of each phaseplate whose slow axis has been already set.

In view of these circumstances, the present invention has been made. Itis an advantage of the invention to provide a liquid crystal displaydevice having a phase plate which can fully exhibit its functions.

Typical aspects of the present invention disclosed herein are brieflydescribed below.

(1) In a liquid crystal display device according to the presentinvention, substrates are disposed on the opposite sides of a liquidcrystal material. A phase plate is formed on the liquid-crystal sidesurface of one of the substrates. The phase plate is covered with colorfilters.

(2) Another liquid crystal display device according to the invention hassubstrates disposed on the opposite sides of a liquid crystal material.The liquid crystal display device also has pixels each having opticallytransmissive and reflective portions. On the liquid-crystal side surfaceof one substrate, a phase plate is formed in the regions of theoptically reflective portions. Color filters are successively formed inthe regions of the optically transmissive and reflective portions fromthe substrate side. A protrusive film is formed in the regions of theoptically reflective portions and in a layer located over the colorfilters. A reflective electrode is formed in regions of the opticallyreflective portions of the liquid-crystal side surface of the othersubstrate.

(3) A further liquid crystal display device according to the inventionis based on, for example, the configuration of the liquid crystaldisplay device (2) above and further characterized in that theprotrusive film and the reflective electrode protrude successivelyincreasing amounts from the peripheries of the phase plate.

(4) A yet other liquid crystal display device according to the inventionis based on, for example, the configuration of the liquid crystaldisplay device (2) above and further characterized in that theprotrusive film and the reflective electrode protrude successivelyincreasing amounts from the peripheries of the phase plate withineffective pixel regions.

(5) A yet further liquid crystal display device according to theinvention is based on, for example, the configuration of the liquidcrystal display device (2) above and further characterized in that thecolor filters are made thinner in portions where the color filtersoverlap the phase plate than in portions where the color filters do notoverlap the phase plate.

It is to be understood that the present invention is not limited tothese configurations and that various changes and modifications can bemade without departing from the technical concept of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a liquid crystal display device accordingto the present invention, taken on line I-I of FIG. 2, showing oneexample of the configuration of the device;

FIG. 2 is a plan view of the liquid crystal display device according tothe invention, showing one example of the pixel structure;

FIGS. 3A and 3B are cross sections, illustrating a process sequence forfabricating a phase plate incorporated in a liquid crystal displaydevice according to the invention;

FIGS. 4A, 4B, and 4C are cross sections, illustrating a process sequencefor fabricating a phase plate incorporated in a liquid crystal displaydevice according to the invention;

FIGS. 5A, 5B, and 5C are cross sections, illustrating a process sequencefor fabricating a phase plate incorporated in a liquid crystal displaydevice according to the invention;

FIGS. 6A, 6B, and 6C are cross sections, illustrating a process sequencefor fabricating a phase plate incorporated in a liquid crystal displaydevice according to the invention;

FIGS. 7A and 7B are plan views showing positional relationships among aphase plate, a protrusive film, a reflective electrode, and a blackmatrix incorporated in a liquid crystal display device according to theinvention;

FIG. 8 is a plan view showing another pixel structure of a liquidcrystal display device according to the invention; and

FIG. 9 is a cross-sectional view taken on line VI-VI of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the liquid crystal display device according to thepresent invention are hereinafter described with reference to thedrawings.

FIG. 2 is a plan view showing one example of the pixel structure of theliquid crystal display device according to the invention. In the liquidcrystal display device, pixels are arranged in a matrix. Only one of thepixels is shown in FIG. 1. Pixels located above and below the shownpixel and pixels located on the left and right sides of the shown pixelare similar in structure with the shown pixel.

The liquid crystal display device according to the present invention isused for color representation. Three adjacent pixels showing colors ofred, green, and blue form one unit of pixel for color representation. InFIG. 1, one pixel showing color of green is shown as an example.

Furthermore, in the liquid crystal display device according to thepresent invention, each pixel has functions of transmitting andreflecting light. A virtual line segment a substantially passes acrossthe center of the pixel region, for example, in the x-direction. Theupper region located over the virtual line segment is formed as anoptically transmissive region TT, while the lower region is formed as anoptically reflective region RT.

Substrates are disposed opposite to each other such that a liquidcrystal material is sandwiched between them. Gate signal lines GLextending in the x- and y-directions are formed on the liquid-crystalside surface of one substrate SUB1 of the two substrates. The gatesignal lines GL surround rectangular regions together with drain signallines DL (described later). Pixels are formed within the regions.

An insulator film (not shown) is formed on the surface of the substrateSUB1 and provides a cover over the gate signal lines GL. The insulatorfilm acts as a gate insulator film in regions where thin-filmtransistors (TFTs) (described later) are formed. The insulator film actsas an interlayer dielectric film at the intersections of the gate signallines GL and the drain signal lines DL.

A semiconductor layer AS is formed in some of locations which are on theupper surface of the insulator film and which overlap the gate signallines GL. The semiconductor layer AS provides the TFTs. Drain electrodesDT and source electrodes ST are formed on the upper surface of thesemiconductor layer AS. As a result, so-called inverted-staggered MIStransistors are built.

The drain electrodes DT and source electrodes ST are formed, forexample, at the same time when the drain signal lines DL are formed.

That is, the drain signal lines DL extend in the y-direction on theupper surface of the insulator film and are juxtaposed in thex-direction. Some of the drain signal lines DL extend so as to be formedon the upper surface of the semiconductor layer AS, thus forming drainelectrodes DT. Source electrodes ST are also formed on the upper surfaceof the semiconductor layer AS and spaced from the drain electrodes DT bya distance equal to the channel length. The source electrodes ST haveextension portions located outside the region where the semiconductorlayer AS is formed, the extension portions extending toward the pixelregions. The extension portions act as connector portions for connectionwith pixel electrodes PX (described later).

A protective film (not shown) is formed on the surface of the substrateSUB1 and covers the drain signal lines DL. The protective film is formedto prevent direct contact of the TFTs with the liquid crystal material.The protective film is a layer of an inorganic material, an organicmaterial, or a lamination thereof.

The pixel electrodes PX are formed on the upper surface of theprotective film. Each of the pixel electrodes PX is made of an opticallytransmissive pixel electrode PXt and an optically reflective pixelelectrode PXr which are electrically connected together.

The optically transmissive pixel electrode PXt is made of ITO, forexample, and is formed over the whole region of the central portion ofthe pixel region excluding slight marginal portions. The opticallyreflective pixel electrode PXr is made of Al, for example, and is formedon the upper surface of the optically transmissive pixel electrode PXtand in the region under the virtual line segment a.

Thus, where each pixel electrode PX is viewed two-dimensionally, it ispartitioned into the region where the optically transmissive pixelelectrode PXt is exposed in the pixel region and the region where theoptically reflective pixel electrode PXr is exposed. The boundary isdelineated by the virtual line segment a.

The substrate SUB1 and another substrate SUB2 are disposed opposite toeach other with a liquid crystal material sandwiched therebetween. Ablack matrix BM is formed on the liquid crystal side of the substrateSUB2 such that the pixel regions are optically shielded at theirperipheries (i.e., at the gate signal lines GL, their vicinities, drainsignal lines DL, and their vicinities). Therefore, regions PT(hereinafter referred to as the effective pixel regions PT) functionsubstantially as pixel regions is the openings of the black matrix(indicated by the solid line in FIG. 2).

Therefore, the region in which the optically transmissive pixelelectrode PXt that is observed within the effective pixel region PT isformed as optically transmissive region TT. The region in which theoptically reflective pixel electrode PXr is formed is formed asoptically reflective region RT.

An orientation film (not shown) is formed on the upper surface of thesubstrate SUB1 and covers the optically transmissive pixel electrodesPXt and optically reflective pixel electrodes PXr. The orientation filmis in direct contact with the liquid crystal material and limits theinitial direction of orientation of the molecules in the liquid crystalmaterial.

A polarizer plate POL1 is formed on the surface of the substrate SUB1that is on the opposite side of the liquid crystal material to visualizethe behavior of the liquid crystal material.

The structure of the substrate SUB2 positioned opposite to the substrateSUB1 with the liquid crystal material LC sandwiched therebetween is nextdescribed by referring to FIG. 1, which is a cross section taken alongline I-I of FIG. 2. The liquid crystal side of the substrate SUB1 shownin FIG. 1 is constructed as follows. Only optically transmissive pixelelectrode PXt, optically reflective pixel electrode PXr, and orientationfilm ROI1 formed so as to cover these electrodes are shown as thestructure of the liquid crystal side of the substrate SUB1. To describethe structure of the liquid crystal side of the substrate SUB2,positional relationships with the optically transmissive pixel electrodePXt and optical reflective pixel electrode PXr are required to be known.Therefore, the gate signal lines GL, insulator film, and protective filmare omitted.

The black matrix BM is formed on the liquid-crystal side surface of thesubstrate SUB2. The black matrix BM has been already described inrelation to the structure shown in FIG. 2. The black matrix is formed toisolate each pixel region from adjacent pixel regions. Hence, theregions within the openings of the black matrix BM are referred to asthe effective pixel regions PT as described previously.

As another embodiment, portions of the black matrix BM (e.g., portionsformed along the drain signal lines DL) may be made of a differentmaterial and mounted on the side of the substrate SUB1. Also, in thiscase, the viewer can recognize regions surrounded by the light-shieldingfilm including the black matrix BM as pixels because when the liquidcrystal display device is driven, the regions in which brightness can bevaried can be identified as the effective pixel regions PT.

A phase plate PL is formed in a region which is located inside the blackmatrix BM and which is opposite to the optically reflective pixelelectrodes PXr.

This phase plate PL acts, for example, as ½-λ wave plate, and is used toprevent the phase of light from shifting by ¼ λ when the light isreflected by the pixel electrodes PXr, the light reciprocating throughthe phase plate PL.

A color filter FIL is formed on the upper surface of the phase plate PLand covers the phase plate. The filter FIL is green, for example, and isformed on all the effective pixel regions PT. Therefore, marginalportions of the color filter FIL are formed to overlap the black matrixBM. The color filter FIL for green is formed on the black matrix BM inback to back relation to the color filters FIL for red and blue,respectively, which are formed at the other adjacent pixels.

Since each color filter FIL is formed to cover the phase plate PL asdescribed previously, the color filter has portions covering the phaseplate PL (optically reflective regions RT) and portions not covering thephase plate PL (optically transmissive regions TT) within the same pixelregion. The color filter is thicker in the optically transmissiveregions TT and thinner in the optically reflective regions RT.

Light passes through each optically transmissive region TT once butpasses through each optically reflective region RT twice. Consequently,the optical distance for the light passing through the color filter FILcan be made substantially equivalent. This effect can be obtainedwithout increasing the number of manufacturing steps. This effect meansthat if the same pixel region is used either in transmissive mode orreflective mode, color tones exhibited by the color filter FIL are notaffected.

Since no orientation film is formed on the upper surface of the phaseplate PL, it is possible to prevent the undesirable situation where thephase plate PL is melted by the liquid solution during formation of theorientation film.

A planarization film OC made of a resin is formed on the upper surfaceon which the color filter FIL is formed. This planarization film OCmakes less conspicuous steps, on the side of the liquid crystal materialLC, created by the formation of the color filter FIL underlying theplanarization film OC.

An optically transparent counter electrode CT is formed from ITO, forexample, on the upper surface of the planarization film OC. The counterelectrode CT is applied with a voltage providing a basis to the signalsupplied to the pixel electrodes PX. A voltage corresponding to thesignal is produced between the counter electrode and the pixelelectrodes PX. An electric field corresponding to the voltage isproduced across the liquid crystal material LC, inducing behavior of theliquid crystal material.

An orientation film ORI2 determining the initial direction oforientation of the molecules of the liquid crystal material is alsoformed on the upper surface of the counter electrode CT. A polarizerplate POL2 is formed on the surface of the substrate SUB2 that is on theopposite side of the liquid crystal material to visualize the behaviorof the liquid crystal material.

The two-dimensional positional relationship among the pixel electrodePXr, protrusive film MR, and phase plate Pl of the liquid crystaldisplay device fabricated as described above is as shown in FIG. 7A.

In this figure, the protrusive film MR and reflective electrode PXrprotrude from the phase plate PL. The protrusive film MR protrudes anamount w′ from the phase plate PL. The reflective electrode PXrprotrudes an amount w from the phase plate PL. The relation w>w′ holds.That is, the protrusive film MR and reflective electrode PXr protrudesuccessively greater amounts from the phase plate PL at theirperipheries to reduce leakage of light from the optically reflectiveregions RT.

Accordingly, if this relation is satisfied within each effective pixelregion PT, satisfactory results will be obtained. This concept leads toa further embodiment as shown in FIG. 7B. That is, the opticallyreflective region RT is placed in the center of the pixel region. Theoptically transmissive region TT is formed around it. The pixelelectrode PXr, protrusive film MR, and phase plate PL need to satisfythe above-described relationship over the whole region of theirperipheries.

In the liquid crystal display device of the structure described above,the black matrix BM, phase plate PL, color filters FIL, andplanarization film OC are formed, for example, in this order on theliquid-crystal side surface of the substrate SUB2. One example of methodof fabricating the phase plate PL is described now.

FIGS. 3A and 3B illustrate a process sequence for forming theorientation film ORI for formation of the phase plate. FIG. 3A is aschematic cross section. FIG. 3B is a schematic plan view.

First, as shown in FIG. 3A, the substrate SUB2 is prepared. The blackmatrix BM has been already formed on the liquid-crystal side surface ofthe substrate SUB2. The orientation film ORI for orientation control isapplied to the same surface and rubbed. This rubbing operation iscarried out, for example, by rotating a rubbing roll RL on the uppersurface of the orientation film ORI as shown.

Then, a UV curable liquid crystal material MM (such as RMS03-001available from Merck) forming the phase plate is applied. FIG. 4A is aschematic cross section. FIG. 4B is a schematic plan view. By formingthe liquid crystal material MM on the rubbed orientation film ORI, themolecules of the material are oriented in accordance with theorientation direction RUB of the orientation film ORI. FIG. 4C is aschematic plan view showing the manner in which the molecules of thematerial MM are oriented in the orientation direction RUB.

Then, the UV curable liquid crystal material MM is selectivelyirradiated with UV light via a mask MK, changing the material into afilm. FIG. 5A is a schematic cross section. FIG. 5B is a schematic planview. Selective exposure to UV light is performed, for example, atportions which are left as the phase plate PL. The portions irradiatedwith UV light undergo photopolymerization. The phase plate PL which isformed by the formation of the film has a phase axis extending along thedirection of rubbing RUB as schematically shown in FIG. 5C.

The phase plate PL is then developed, thus performing patterning. FIG.6A is a schematic cross section. FIG. 6B is a schematic plan view. FIG.6C is a schematic plan view, showing the manner in which the phase axishas been formed.

As a subsequent process step, the color filter FIL is formed.

In the liquid crystal display device shown in FIG. 2, the counterelectrode CT is formed on the side of the substrate SUB2. Obviously, thecounter electrode may be formed on the side of the substrate SUB1together with the pixel electrodes PX.

In this case, a structure as shown in FIG. 8 is adopted as oneembodiment. FIG. 8 is drawn in a corresponding manner to FIG. 2, forexample. The arrangement of gate signal lines GL, drain signal lines DL,and TFTs and their structures are similar with the arrangement andstructures shown in FIG. 1. The differences with the structures shown inFIG. 2 are the pixel electrodes PX and a newly prepared counterelectrode CT.

In FIG. 8, an optically transmissive counter electrode CTt and anoptically reflective counter electrode CTr are successively overlappedin each pixel region of the substrate SUB1.

The electrodes are patterned such that the counter electrode CTt isexposed in the optically transmissive region TT and that the counterelectrode CTr is exposed in the optically reflective region RT. Thecounter electrodes CT (i.e., CTt and CTr) are electrically connectedwith counter voltage signal lines CL which are close and parallel, forexample, to the gate signal lines GL. A reference voltage is applied viathe counter voltage signal lines CL. The counter voltage signal lines CLare formed at the same time, for example, as when the gate signal linesGL are formed. The signal lines CL and GL are both made of the samematerial.

On the other hand, the pixel electrodes PX are formed as a layer over aninsulator film or films. The counter electrodes CT are positioned underthe insulator film. For example, the pixel electrodes are made ofbelt-like electrodes which extend in the y-direction and are juxtaposedin the x-direction.

The electrodes are connected together, for example, at ends of the TFTs,and are connected with the source electrodes ST of the TFTs throughcontact holes formed in the insulator film. Although the numericalaperture of the pixels can be improved by forming the pixel electrodesPX from an optically transparent material, the material is not limitedto it. A non-transparent material may also be used.

When signals supplied via the TFTs are applied to the pixel electrodesPX, an electric field is produced between each pixel electrode and thecounter electrode, causing behavior of the liquid crystal material.

In this case, the electric field is principally substantially parallelto the surface of the substrate SUB1. Besides, an electric field isproduced vertical to the edges of the pixel electrodes PX. Thus, theliquid crystal material LC is driven.

A cross section taken on line VI-VI of FIG. 8 is shown in FIG. 9, whichis drawn in a corresponding manner to FIG. 1. The configuration on theside of the substrate SUB2 is similar to the configuration shown inFIG. 1. That is, black matrix BM, phase plate PL, color filters FIL,protrusive film MR, and other components are formed, using the effectivepixel regions PT, optically reflective regions RT, and opticallytransmissive regions TT as a reference.

The embodiments described above can be used alone or in combinationbecause the advantages of each embodiment can be exhibited alone or incombination.

In the liquid crystal display device in the present embodiment, eachpixel has both optically transmissive and optically reflective regions.Obviously, the present invention can also be applied to a structure inwhich the optically transmissive and optically reflective regions areassigned to the pixels.

Furthermore, the invention is not limited to this structure. Of course,the invention can be applied to every kind of liquid crystal displaydevice having a built-in phase plate. In this case, the invention can beapplied if the functions of this phase plate are slightly different fromthose of the aforementioned phase plate.

1. A liquid crystal display device comprising: a first substrate and asecond substrate disposed opposite to each other with a liquid crystalmaterial sandwiched therebetween; and a phase plate formed between thesecond substrate and the liquid crystal material; wherein the phaseplate is coated with color filters which are at least partially formedbetween the phase plate and the liquid crystal material.
 2. A liquidcrystal display device comprising: a first substrate and a secondsubstrate disposed opposite to each other with a liquid crystal materialsandwiched therebetween; pixels each having optically transmissive andoptically reflective portions; a phase plate formed between the secondsubstrate and the liquid crystal material and in regions of theoptically reflective portions; color filters formed between the secondsubstrate and the liquid crystal material in regions of the opticallytransmissive portions and between the phase plate and the liquid crystalmaterial in regions of the optically reflective portions; a protrusivefilm formed in a layer located between the color filters and the liquidcrystal material in the regions of the optically reflective portions;and a reflective electrode formed between the first substrate and theliquid crystal material in regions of the optically reflective portions.3. A liquid crystal display device as set forth in claim 2, wherein theprotrusive film and the reflective electrode protrude successivelyincreasing amounts at their peripheries from the phase plate.
 4. Aliquid crystal display device as set forth in claim 2, wherein theprotrusive film and the reflective electrode protrude large amounts fromthe phase plate within effective pixel regions.
 5. A liquid crystaldisplay device as set forth in claim 2, wherein the color filters aremade thinner in portions where the color filters overlap the phase platethan in portions where the color filters do not overlap the phase plate.